A mixer refers to an electronic device that converts an input signal with a frequency of f1 and an input signal with a frequency of f2 to an output signal having characteristic frequencies of f1+f2 and f1−f2. Through the mixer, the frequency of an input signal can be shifted to a higher frequency or a lower frequency value, so as to facilitate signal processing. For example, the frequency of a signal can be shifted by a mixer, in order to separate the signal from a noise signal, then noise can be filtered out using a filter, and the frequency of the signal is then shifted back to the original value using a mixer, in order to provide noise signal processing. Therefore, the mixers are widely used in signal processing circuit technologies.
Mixers used currently include two types, i.e., a passive type and an active type. The passive mixer uses one or more diodes, and achieves a multiplying operation by using nonlinear segments of the current-voltage characteristic curve of the diodes which has an approximately quadratic character, in operation, the sum of two input signals acts upon the diodes, and an output signal including the product of the two input signals can be obtained by further converting the diode current signals output to voltage signals.
The active mixer uses a multiplier (e.g., a transistor or a vacuum tube) to increase strength of the product signal, by mixing an input signal at a frequency with a local frequency, producing an output signal with frequency components including the addition and subtraction of the two frequencies, and the active mixer increases the degree of isolation of two input ends, but it may have higher noise and greater power consumption.
The above mixers have the following problems:
1) The diode mixer uses an approximate processing method, such that the output signal, in addition to including a desired frequency, also has other frequencies, and these have signal strength that is relatively great; it is thus necessary to subsequently use a filter or other technologies to separate the interference in order to provide the desired signal.
2) The active mixer achieves frequency mixing by using a local oscillator, the output signal includes many other frequencies, it is also necessary to use a filter for separation, and it is necessary to use a multiplier, a local oscillator and other devices, which increases complexity and power consumption of the circuit.
3) The input signals and the output signals cannot be effectively isolated, and they will affect each other.